1. Field of the Invention
The field of the invention is that of data transmission in packet mode, on any type of network and, notably, on relatively complex networks, bringing into play the notion of a virtual circuit between two transmitter/receiver terminals in a state of connection.
More precisely, the invention relates to a method of transmission by packets, implementing an adaptive data-compression algorithm.
The invention can be applied notably but not exclusively to data exchanges according to the protocol X25, standardized by the CCITT in 1984. In this case, and more generally whenever it is necessary to use a public communications network, the user has to pay a fee which depends, for example, on the type of line allocated and/or the time for which the line is used. This is notably the case with special-purpose lines allocated by the public services, the cost of which is proportional to the maximum bit rate of the line.
2. Description of the Prior Art
There already are many known systems, in the field of data transmission, implementing adaptive compression mechanisms. However, these systems relate only to point-to-point or point-multipoint links but not to complete communications networks.
Adaptive compression algorithms evolve dynamically as a function of the characteristics of the data stream, unlike static algorithms which use pre-established correspondence tables. These adaptive compression algorithms can, for example, learn the coding rules by analyzing the frequency of the characters or strings of characters, and store them in a constantly updated "dictionary". For a given direction of transmission, an adaptive compression algorithm has to be implemented both on the compressor side and on the decompressor side in a manner that is parallel and perfectly synchronized so that, at any time, the ends have strictly identical dictionaries. These algorithms, which are more complex than static algorithms, also perform better. An algorithm of this type, known as a "Ziv Lempel" algorithm, has been chosen for example in the CCITT recommendation V42a for asynchronous modems.
In the case of transmission by packets, for example according to the standard X25, many difficulties arise for the implementation of such compression systems.
First of all, the absence of a standard makes it impossible, at present, to perform the compression directly in the terminals. It is therefore necessary to transfer this compression to the network units. As a rule, a link (the word "link" is used herein to mean an exchange between two terminals) between two terminals goes through a set of intermediate nodes. The transfer of the compression to the network units, at least when the problem is initially analyzed, dictates the repetition of the decompression and compression operation in each of these intermediate nodes.
Besides, it must be noted that each of the network units most usually manage several communications simultaneously. This means that, to carry out the decompression and compression operations simultaneously, it is necessary to have substantial memorizing and processing means, dimensioned so as to cope with the maximum possible number of simultaneous links.
It is an aim of the invention, notably, to overcome the different drawbacks of the prior art in meeting the different constraints dictated by transmission in packets.